Adsorption of Phenol from Dilute and Concentrated Aqueous Solutions by Activated Carbons

Abstract

IntroductionThe adsorption of phenol from aqueous solutions onto carbons has received a great deal of attention, and an exhaustive review has been published recently by Radovic et al. 1 At the present time, the underlying mechanism and the prediction of adsorption equilibrium remain open questions, although a number of models have been proposed. It appears that the pH of the solution, the real surface area of the solid, and functional groups play a major role. A majority of authors describe the overall adsorption equilibrium in terms of Langmuir, Freundlich, or Redlich isotherms, and correlations have recently been suggested between the basic parameters of the isotherm and more fundamental properties such as the chemistry of the surface. Some authors have also attempted to correlate adsorption equilibrium with thermodynamic properties, by using calorimetry. 2-5 However, a major drawback of the classical models (Langmuir, Freundlich, etc.) is the difficulty in predicting adsorption equilibrium based on simple physicochemical parameters. As pointed out earlier and discussed in detail in a recent study by Stoeckli et al., 2,3 it appears that an adaptation of Dubinin's theory to the solid-liquid equilibrium provides an interesting framework for the description of phenol adsorption from aqueous solutions. A major advantage of this approach is the fact that adsorption can be predicted over a temperature range of 20-30°around room temperature, on the basis of simple physicochemical parameters and structural characteristics.It has been shown 3 that in the case of carbons with low oxygen contents, adsorption of phenol from dilute aqueous solutions corresponds essentially to the coating of the external surface and of the micropore walls by a monolayer. A good correlation was also obtained with the corresponding enthalpies of immersion at 293 K, based on a specific enthalpy near -0.110 J m -2 . On the other hand, a recent study by Stoeckli and Hugi-Cleary 2 on the adsorption of phenol from concentrated solutions (phenol liquefied with 15-25% water w/w) suggests that the mechanism corresponds to the filling of the micropore volume of activated carbons. This unambiguous result is provided by immersion calorimetry at 293 K, with benzene as a reference and combined with the adsorption of phenol from the vapor phase. Both types of experiment suggest an affinity coefficient (phenol) close to unity.It appears therefore that the adsorption of phenol, and possibly of its derivatives, follows two distinct mechanisms, and the next step, presented in this study, deals with the influence of the surface chemistry on these two mechanisms. We considered essentially the effect of surface oxygen [O], which can vary between less than 1 and 10 mmol g -1 . In view of their small concentrations (usually less than 1-1.5 mequiv g -1 ), the basic surface groups, identified by HCl titration, 6,7 were not considered. As shown below, in both concentrated and dilute solutions, it appears that water is preferentially adsorbed by the oxygen-containi...

Modelling of water adsorption by activated carbons: effects of microporous structure and oxygen content

“…This approach follows the same lines as previously [6], but includes the specific interactions between water and variable amounts of surface oxygen, as well as a limited number of basic sites, characterized by HCl titration. As shown earlier, basic sites contain little oxygen [3], but immersion calorimetry [3,[5][6][7]13] indicates that their specific interactions with water are practically the same as those observed on average for the oxygen atoms in surface complexes ()10 kJ mol À1 ). This means that these sites, which do not necessarily include all basic sites, can be considered as hydrophilic centres and may be included in calculations as equivalent oxygen sites.…”

“…Moreover, as illustrated by A ¼ RT lnðp s =pÞ. This means that the requirement for temperature invariance, postulated by Dubinin's theory and observed experimentally for water adsorption by microporous carbons with variable degrees of oxidation [3,[5][6][7]20], is also fulfilled by modelling. Basically, this indicates that modelling validates the application of Dubinin's theory to water adsorption suggested some years ago by Stoeckli et al [18].…”

“…The fundamental aspects have been presented in detail in the first paper [8] and in the recent study by Stoeckli [3] and consequently we shall limit ourselves to the essential.…”

“…A small correction has to be applied for the wetting of the external (non-microporous) surface S e of the carbon. Finally, studies based on the correlation between E o and the average micropore-width [21], the enthalpy of immersion of phenol from dilute aqueous solutions [3] and the modelling of CO 2 adsorption [22], show that the actual surface area of the micropore walls S mi can be estimated with a good approximation.…”

“…In such processes, water vapour is also present at relative pressures above 0.5-0.6 and its influence on mixed adsorption depends strongly on the amount of oxygen present on the surface. Typical and untreated activated carbons contain less than 1.5-2 mmol of oxygen per gram [2], which corresponds to a small fraction of the real surface area of the micropore walls [3]. Under these circumstances, the water adsorption isotherm is almost of type V [4][5][6][7], but selective treatments may increase the amount of oxygen.…”